FIG. 11 shows an example of an exposure apparatus, which exposes a substrate, such as a wafer, or the like, to a pattern. In FIG. 11, exposure light 102 emitted from a laser oscillator 101 as an exposure light source is guided to a wafer 111 placed on a wafer stage 112 through reflection mirrors 103 and 108, relay lenses 106 and 107, a reticle 109, and a projection optical system 110.
Conventionally, to measure a change in reflectance of the reflection mirror 103, exposure is temporarily stopped, and an intensity monitor 104 attached to a retractable mechanism 105 is inserted to an optical path, thereby performing measurement.
For this reason, while the reflectance of an optical device, such as the reflection mirror 103 is measured, the wafer 111 cannot be exposed to a pattern.